Phosphates are one of several classes of compounds which are added to cellulose ester film and molded materials to improve commercially valuable properties. It is well known in the art that certain phosphorus containing compounds, such as phosphates, can act as flame retardants for many polymers including cellulose esters. It is also known that certain phosphorus containing compounds can function as plasticizers for various polymers including cellulose esters. Generally plasticization lowers the dimensional stability of the extruded sheet or film under conditions of increasing temperature. In some production situations the volatility of the phosphate causes major technical difficulties. Volatilization can change the composition of the phosphate in the product, thereby changing the properties of the product; and recondensation of phosphate vapors can lead to production difficulties and defects in the final product. Thus, a plasticizer which has low volatility and makes the material flame-retardant more dimensionally stable and more formulaically stable is of considerable value.
The effectiveness of certain phosphorus containing materials, especially triphenyl phosphate, as flame-retardants for plastics is well known (R. M. Aseeva and G. E. Zaikov, in Adv. in Polymer Science, 70, p. 217, 1985). In European Patent Application 264143, bisphosphates were claimed as flame-retardants for polyesters, but this reference does not teach application to cellulose esters. U.S. Pat. No. 2,782,128 and Kokai Patent No. SHO 60[1985]-25003 describe the use of bisphosphates with cellulose esters but the structures of these disclosed bisphosphates all have alkyl pendant groups with either alkylene bridges or monooxaalkylene bridges. None of these references teach aryl pendant groups with arylene bridges. U.S. Pat. No. 4,388,431 also teaches bisphosphates in combination with cellulose esters but these bisphosphates are all aliphatic diol cyclic phosphate esters and not diaryl phosphate structures. A related British Patent 1,598,744 claims a long list of monomeric or polymeric compounds which contain aliphatically- or aromatically-bound halogen as well as cyclic phosphates and thionophosphates as flame-retardants at not more than 5 weight percent addition.
It should be noted that most of the uses for cellulose esters require good clarity. Many of the phosphate components taught in the art for use with cellulose esters will not result in sufficient clarity. It is, therefore, apparent that a flame-retarding, formulaically stabilizing, dimensionally stabilizing plasticizer for both film and molded materials produced from cellulose esters is desirable. As judged by the clarity of the final solid product, this additive should be compatible with cellulose esters. Also, it would be desirable if the additives improved the thermal or mechanical properties of the film or molded product relative to prior art additives.